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          <th colspan="3" align="center">BTree Configuration</th>
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          <td width="20%" align="left"><a accesskey="p" href="cachesize.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 6. Database Configuration</th>
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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="btree"></a>BTree Configuration</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="btree.html#duplicateRecords">Allowing Duplicate Records</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="btree.html#comparators">Setting Comparison Functions</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        In going through the previous chapters in this book, you may notice that
        we touch on some topics that are specific to BTree, but we do not cover
        those topics in any real detail. In this section, we will discuss
        configuration issues that are unique to BTree.
    </p>
      <p>
        Specifically, in this section we describe:      
    </p>
      <div class="itemizedlist">
        <ul type="disc">
          <li>
            <p>
                Allowing duplicate records.
            </p>
          </li>
          <li>
            <p>
                Setting comparator callbacks.
            </p>
          </li>
        </ul>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="duplicateRecords"></a>Allowing Duplicate Records</h3>
            </div>
          </div>
        </div>
        <p>
            BTree databases can contain duplicate records. One record is
            considered to be a duplicate of another when both records use keys
            that compare as equal to one another.
        </p>
        <p>
            By default, keys are compared using a lexicographical comparison,
            with shorter keys collating higher than longer keys.
            You can override this default using the
                
                <code class="methodname">Db::set_bt_compare()</code>
                
            method. See the next section for details.
        </p>
        <p>
            By default, DB databases do not allow duplicate records. As a
            result, any attempt to write a record that uses a key equal to a
            previously existing record results in the previously existing record
            being overwritten by the new record.
        </p>
        <p>
            Allowing duplicate records is useful if you have a database that
            contains records keyed by a commonly occurring piece of information.
            It is frequently necessary to allow duplicate records for secondary
            databases.
         </p>
        <p>
            For example, suppose your primary database contained records related
            to automobiles. You might in this case want to be able to find all
            the automobiles in the database that are of a particular color, so
            you would index on the color of the automobile. However, for any
            given color there will probably be multiple automobiles. Since the
            index is the secondary key, this means that multiple secondary
            database records will share the same key, and so the secondary
            database must support duplicate records.
        </p>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="sorteddups"></a>Sorted Duplicates</h4>
              </div>
            </div>
          </div>
          <p>
                Duplicate records can be stored in sorted or unsorted order. 
                You can cause DB to automatically sort your duplicate
                records by 
                <span> 
                    specifying the <code class="literal">DB_DUPSORT</code> flag at
                    database creation time.
                </span>
                
            </p>
          <p>
                If sorted duplicates are supported, then the 
                <span>
                    sorting function specified on 
                    
                    <code class="methodname">Db::set_dup_compare()</code>
                </span>
                
                is used to determine the location of the duplicate record in its
                duplicate set. If no such function is provided, then the default
                lexicographical comparison is used.
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="nosorteddups"></a>Unsorted Duplicates</h4>
              </div>
            </div>
          </div>
          <p>
                For performance reasons, BTrees should always contain sorted
                records. (BTrees containing unsorted entries must potentially 
                spend a great deal more time locating an entry than does a BTree
                that contains sorted entries).  That said, DB provides support 
                for suppressing automatic sorting of duplicate records because it may be that
                your application is inserting records that are already in a
                sorted order.
            </p>
          <p>
                That is, if the database is configured to support unsorted
                duplicates, then the assumption is that your application
                will manually perform the sorting. In this event,
                expect to pay a significant performance penalty. Any time you
                place records into the database in a sort order not know to
                DB, you will pay a performance penalty
            </p>
          <p>
                That said, this is how DB behaves when inserting records
                into a database that supports non-sorted duplicates:
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                    If your application simply adds a duplicate record using 
                        
                        <span><code class="methodname">Db::put()</code>,</span>
                        
                    then the record is inserted at the end of its sorted duplicate set.
                </p>
              </li>
              <li>
                <p>
                    If a cursor is used to put the duplicate record to the database,
                    then the new record is placed in the duplicate set according to the
                    flags that are provided on the 
                        
                        <code class="methodname">Dbc::put()</code>
                    method. The relevant flags are:
                </p>
                <div class="itemizedlist">
                  <ul type="circle">
                    <li>
                      <p>
                            <code class="literal">DB_AFTER</code>
                            
                        </p>
                      <p>
                        The data
                        <span>
                            provided on the call to
                            
                            <code class="methodname">Dbc::put()</code>
                        </span>
                        is placed into the database
                        as a duplicate record. The key used for this operation is
                        the key used for the record to which the cursor currently
                        refers. Any key provided on the call 
                        
                        <span>
                        to
                            
                            <code class="methodname">Dbc::put()</code>
                        </span>

                        is therefore ignored.
                        </p>
                      <p>
                            The duplicate record is inserted into the database
                            immediately after the cursor's current position in the
                            database.
                        </p>
                      <p>
                            This flag is ignored if sorted duplicates are supported for
                            the database.
                        </p>
                    </li>
                    <li>
                      <p>
                            <code class="literal">DB_BEFORE</code>
                            
                        </p>
                      <p>
                            Behaves the same as 
                                <code class="literal">DB_AFTER</code>
                                
                            except that the new record is inserted immediately before 
                            the cursor's current location in the database.
                        </p>
                    </li>
                    <li>
                      <p>
                            <code class="literal">DB_KEYFIRST</code>
                            
                        </p>
                      <p>
                            If the key 
                            <span>
                                provided on the call to
                                
                                <code class="methodname">Dbc::put()</code>
                            </span>
                            already exists in the
                            database, and the database is configured to use duplicates
                            without sorting, then the new record is inserted as the first entry
                            in the appropriate duplicates list.
                        </p>
                    </li>
                    <li>
                      <p>
                            <code class="literal">DB_KEYLAST</code>
                            
                        </p>
                      <p>
                            Behaves identically to
                                <code class="literal">DB_KEYFIRST</code>
                                
                            except that the new duplicate record is inserted as the last
                            record in the duplicates list.
                        </p>
                    </li>
                  </ul>
                </div>
              </li>
            </ul>
          </div>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="specifyingDups"></a>Configuring a Database to Support Duplicates</h4>
              </div>
            </div>
          </div>
          <p>
            Duplicates support can only be configured
            at database creation time. You do this by specifying the appropriate
            <span>
                flags to
                
                <code class="methodname">Db::set_flags()</code>
            </span>
            
            before the database is opened for the first time.
        </p>
          <p>
            The 
                <span>flags</span>
                
            that you can use are:
        </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                    <code class="literal">DB_DUP</code>
                    
                </p>
                <p>
                    The database supports non-sorted duplicate records.
                </p>
              </li>
              <li>
                <p>
                    <code class="literal">DB_DUPSORT</code>
                    
                </p>
                <p>
                    The database supports sorted duplicate records. Note
                    that this flag also sets the 
                    <code class="literal">DB_DUP</code> flag for
                    you.
                </p>
              </li>
            </ul>
          </div>
          <p>
            The following code fragment illustrates how to configure a database
            to support sorted duplicate records:
        </p>
          <a id="cxx_btree_dupsort"></a>
          <pre class="programlisting">#include &lt;db_cxx.h&gt;
...

Db db(NULL, 0);
const char *file_name = "myd.db";

try {
    // Configure the database for sorted duplicates
    db.set_flags(DB_DUPSORT);

    // Now open the database
    db.open(NULL,       // Txn pointer
            file_name,  // File name
            NULL,       // Logical db name (unneeded)
            DB_BTREE,   // Database type (using btree)
            DB_CREATE,  // Open flags
            0);         // File mode. Using defaults
} catch(DbException &amp;e) {
    db.err(e.get_errno(), "Database '%s' open failed.", file_name);
} catch(std::exception &amp;e) {
    db.errx("Error opening database: %s : %s\n", file_name, e.what());
} 

...

try {
    db.close(0);
} catch(DbException &amp;e) {
    db.err(e.get_errno(), "Database '%s' close failed.", file_name);
} catch(std::exception &amp;e) {
    db.errx("Error closing database: %s : %s\n", file_name, e.what());
} 

</pre>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="comparators"></a>Setting Comparison Functions</h3>
            </div>
          </div>
        </div>
        <p>
            By default, DB uses a lexicographical comparison function where
            shorter records collate before longer records. For the majority of
            cases, this comparison works well and you do not need to manage
            it in any way. 
         </p>
        <p>
            However, in some situations your application's performance can
            benefit from setting a custom comparison routine. You can do this
            either for database keys, or for the data if your
            database supports sorted duplicate records.
         </p>
        <p>
            Some of the reasons why you may want to provide a custom sorting
            function are:
         </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                    Your database is keyed using strings and you want to provide
                    some sort of language-sensitive ordering to that data. Doing
                    so can help increase the locality of reference that allows
                    your database to perform at its best.
                </p>
            </li>
            <li>
              <p>
                    You are using a little-endian system (such as x86) and you
                    are using integers as your database's keys. Berkeley DB
                    stores keys as byte strings and little-endian integers
                    do not sort well when viewed as byte strings. There are
                    several solutions to this problem, one being to provide a
                    custom comparison function. See
                    <a class="ulink" href="http://download.oracle.com/docs/cd/E17076_02/html/programmer_reference/am_misc_faq.html" target="_top">http://download.oracle.com/docs/cd/E17076_02/html/programmer_reference/am_misc_faq.html</a> 
                    for more information.
                </p>
            </li>
            <li>
              <p>
                    You you do not want the entire key to participate in the
                    comparison, for whatever reason.  In 
                    this case, you may want to provide a custom comparison
                    function so that only the relevant bytes are examined.
                </p>
            </li>
          </ul>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="creatingComparisonFunctions"></a>
                <span>Creating Comparison Functions</span>
                
            </h4>
              </div>
            </div>
          </div>
          <p>
                You set a BTree's key
                    <span>
                        comparison function 
                    </span>
                    
                using
                    
                    <span><code class="methodname">Db::set_bt_compare()</code>.</span>
                    
                You can also set a BTree's duplicate data comparison function using
                    
                    <span><code class="methodname">Db::set_dup_compare()</code>.</span>
                    
                
            </p>
          <p>
            <span>
            You cannot use these methods after the database has been opened.
            Also, if 
            </span>
            
            the database already exists when it is opened, the
                    <span>
                        function 
                    </span>
                    
            provided to these methods must be the same as
            that historically used to create the database or corruption can
            occur.
         </p>
          <p>
                The value that you provide to the <code class="methodname">set_bt_compare()</code> method 
                is a pointer to a function that has the following signature:
            </p>
          <pre class="programlisting">int (*function)(Db *db, const Dbt *key1, const Dbt *key2)</pre>
          <p>
                This function must return an integer value less than, equal to,
                or greater than 0. If key1 is considered to be greater than
                key2, then the function must return a value that is greater than
                0. If the two are equal, then the function must return 0, and if
                the first key is less than the second then the function must return
                a negative value.
            </p>
          <p>
                The function that you provide to <code class="methodname">set_dup_compare()</code> 
                works in exactly the same way, except that the 
                 
                <code class="literal">Dbt</code> 
                parameters hold record data items instead of keys.
            </p>
          <p>
                For example, an example routine that is used to sort integer
                keys in the database is:
                
            </p>
          <a id="cxx_btree1"></a>
          <pre class="programlisting">int
compare_int(Db *dbp, const Dbt *a, const Dbt *b)
{
    int ai, bi;

    // Returns: 
    // &lt; 0 if a &lt; b 
    // = 0 if a = b 
    // &gt; 0 if a &gt; b 
    memcpy(&amp;ai, a-&gt;get_data(), sizeof(int)); 
    memcpy(&amp;bi, b-&gt;get_data(), sizeof(int)); 
    return (ai - bi); 
} </pre>
          <p>
            Note that the data must first be copied into memory that is
            appropriately aligned, as Berkeley DB does not guarantee any kind of
            alignment of the underlying data, including for comparison routines.
            When writing comparison routines, remember that databases created on
            machines of different architectures may have different integer byte
            orders, for which your code may need to compensate.
         </p>
          <p>
            To cause DB to use this comparison function:
         </p>
          <a id="cxx_btree2"></a>
          <pre class="programlisting">#include &lt;db_cxx.h&gt;
#include &lt;string.h&gt;

...

Db db(NULL, 0);

// Set up the btree comparison function for this database
db.set_bt_compare(compare_int);

// Database open call follows sometime after this.</pre>
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